TSL235R
LIGHT TO FREQUENCY CONVERTER
TAOS038 APRIL 2002
1
The
LUMENOLOGY
r
Company
t
t
Copyright
E
2002, TAOS Inc.
www.taosinc.com
D
High-Resolution Conversion of Light
Intensity to Frequency With No External
Components
D
Communicates Directly With a
Microcontroller
D
Compact Three-Leaded Clear-Plastic
Package
D
Single-Supply Operation Down to 2.7 V
D
Nonlinearity Error Typically 0.2% at 100 kHz
D
Stable 150 ppm/
C Temperature Coefficient
D
Single-Supply Operation
Description
The TSL235R light-to-frequency converter combines a silicon photodiode and a current-to-frequency converter
on a single monolithic CMOS integrated circuit. Output is a square wave (50% duty cycle) with frequency directly
proportional to light intensity (irradiance) on the photodiode. The digital output allows direct interface to a
microcontroller or other logic circuitry. The device has been temperature compensated for the
ultraviolet-to-visible light range of 320 nm to 700 nm and responds over the light range of 320 nm to 1050 nm.
The TSL235R is characterized for operation over the temperature range of 25
C to 70
C and is supplied in
a 3-lead clear plastic side-looker package with an integral lens.
Functional Block Diagram
Light
Current-to-Frequency
Converter
Photodiode
Output
t
t
Texas Advanced Optoelectronic Solutions Inc.
800 Jupiter Road, Suite 205
S
Plano, TX 75074
S
(972) 673-0759
3
2
1
OUT
V
DD
GND
PACKAGE
(FRONT VIEW)
TSL235R
LIGHT TO FREQUENCY CONVERTER
TAOS038 APRIL 2002
2
t
t
Copyright
E
2002, TAOS Inc.
The
LUMENOLOGY
r
Company
www.taosinc.com
Absolute Maximum Ratings over operating free-air temperature range (unless otherwise noted)
Supply voltage, V
DD
(see Note 1)
6 V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Operating free-air temperature range, T
A
25
C to 70
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Storage temperature range, T
stg
25
C to 85
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds
240
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Stresses beyond those listed under "absolute maximum ratings" may cause permanent damage to the device. These are stress ratings only, and
functional operation of the device at these or any other conditions beyond those indicated under "recommended operating conditions" is not
implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
NOTE 1: All voltage values are with respect to GND.
Recommended Operating Conditions
MIN
NOM
MAX
UNIT
Supply voltage, V
DD
2.7
5
5.5
V
Operating free-air temperature range, T
A
25
70
C
Electrical Characteristics at V
DD
= 5 V, T
A
= 25
C (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
V
OH
High-level output voltage
I
OH
= 4 mA
4
4.5
V
V
OL
Low-level output voltage
I
OL
= 4 mA
0.25
0.4
V
I
DD
Supply current
2
3
mA
Full-scale frequency
500
kHz
Temperature coefficient of output frequency
Wavelength < 700nm
150
ppm/
C
k
SVS
Supply-voltage sensitivity
V
DD
= 5 V
10%
0.5
%/V
Full-scale frequency is the maximum operating frequency of the device without saturation.
Operating Characteristics at V
DD
= 5 V, T
A
= 25
C
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
f
O
Output frequency
E
e
= 430
W/cm
2
,
p
= 635 nm
200
250
300
kHz
f
O
Output frequency
E
e
= 0
W/cm
2
0.4
10
Hz
N
li
it
f
O
= 0 kHz to 10 kHz
0.1%
%F.S.
Nonlinearity
f
O
= 0 kHz to 100 kHz
0.2%
%F.S.
Step response to full-scale step input
1 pulse of new
frequency plus 1
s
Full-scale frequency is the maximum operating frequency of the device without saturation.
Nonlinearity is defined as the deviation of f
O
from a straight line between zero and full scale, expressed as a percent of full scale.
TSL235R
LIGHT TO FREQUENCY CONVERTER
TAOS038 APRIL 2002
3
The
LUMENOLOGY
r
Company
t
t
Copyright
E
2002, TAOS Inc.
www.taosinc.com
TYPICAL CHARACTERISTICS
1
0.1
0.01
0.001
0.001
0.01
0.1
1
10
10
100
OUTPUT FREQUENCY
vs
IRRADIANCE
100
1 k
E
e
Irradiance
W/cm
2
V
DD
= 5 V
p
= 635 nm
T
A
= 25
C
1000
f
O
-- Output Frequency -- kHz
Figure 1
Figure 2
300
400
500
600
700
1000
800
900
1100
Wavelength nm
Normalized Responsivity
0
0.2
0.4
0.6
0.8
1.0
1.2
PHOTODIODE SPECTRAL RESPONSIVITY
DARK FREQUENCY
vs
TEMPERATURE
V
DD
= 5 V
E
e
= 0
T
A
Temperature
C
Wavelength of Incident Light nm
V
DD
= 5 V
TEMPERATURE COEFFICIENT
OF OUTPUT FREQUENCY
vs
WAVELENGTH OF INCIDENT LIGHT
25
0
25
50
75
0
0.2
0.4
0.6
0.8
1
1.2
0
1000
2000
3000
4000
5000
6000
7000
300
400
500
600
700
1000
800
900
T
emperature
Coefficient of Output Frequency -- ppm/
5
C
f
O(dark)
-- Dark Frequency -- Hz
Figure 3
Figure 4
TSL235R
LIGHT TO FREQUENCY CONVERTER
TAOS038 APRIL 2002
4
t
t
Copyright
E
2002, TAOS Inc.
The
LUMENOLOGY
r
Company
www.taosinc.com
TYPICAL CHARACTERISTICS
OUTPUT FREQUENCY
vs
SUPPLY VOLTAGE
V
DD
Supply Voltage V
Normalized Output Frequency
2.5
3
3.5
4
4.5
5
0.980
0.985
0.990
0.995
1.000
1.005
1.010
5.5
T
A
= 25
C
f
O
= 250 kHz
Figure 5
TSL235R
LIGHT TO FREQUENCY CONVERTER
TAOS038 APRIL 2002
5
The
LUMENOLOGY
r
Company
t
t
Copyright
E
2002, TAOS Inc.
www.taosinc.com
APPLICATION INFORMATION
Power-supply considerations
Power-supply lines must be decoupled by a 0.01-
F to 0.1-
F capacitor with short leads placed close to the
TSL235R (Figure 6).
Output interface
The output of the device is designed to drive a standard TTL or CMOS logic input over short distances. If lines
greater than 12 inches are used on the output, a buffer or line driver is recommended.
Measuring the frequency
The choice of interface and measurement technique depends on the desired resolution and data-acquisition
rate. For maximum data-acquisition rate, period-measurement techniques are used.
Period measurement requires the use of a fast reference clock with available resolution directly related to
reference-clock rate. The technique is employed to measure rapidly varying light levels or to make a fast
measurement of a constant light source.
Maximum resolution and accuracy may be obtained using frequency-measurement, pulse-accumulation,
or integration techniques. Frequency measurements provide the added benefit of averaging out random- or
high-frequency variations (jitter) resulting from noise in the light signal. Resolution is limited mainly by available
counter registers and allowable measurement time. Frequency measurement is well suited for slowly varying
or constant light levels and for reading average light levels over short periods of time. Integration, the
accumulation of pulses over a very long period of time, can be used to measure exposure -- the amount of light
present in an area over a given time period.
TSL235R
Timer/Port
MCU
0.1
F
V
DD
2
3
1
Figure 6. Typical TSL235R Interface to a Microcontroller
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